Sperm antibodies have been found to be present following vasectomy in men, non-human primates and various rodent species. High titers of sperm antibody following vasectomy have been implicated as a causal factor for low pregnancy rates after vasovasostomies in man and primates. A possible solution to this problem would be to prevent formation of sperm antibody production by administration of passive sperm antibody following vasectomy. To test this hypothesis, vasectomy was performed on guinea pigs by standard surgical technique. Animals which received injections of sperm antibody immediately after vasectomy did not produce detectable titers of sperm antibody, while untreated animals developed high titers of sperm antibody. Our data demonstrates that administration of passive antibody does prevent the development of sperm antibody following vasectomy. The proposed experimental plan is to determine if the procedure is of clinical significance. a. Cynomolgus monkeys are vasectomized. Part of the animals are treated with passive monkey sperm antibody. The sperm antibody in all animals is determined for 6 months. All animals are vasovasostomized. The success of the treatment is determined by the level of sperm in the ejaculate and reproductive success of the animals with fertile females. b. Determine how long active sperm antibody synthesis can be suppressed following vasectomy by administration of passive sperm antibody. To be a useful procedure, treatment with passive antibody must prevent active immunization to sperm for a number of years. The administration of passive antibody prevents antibody development over the time interval we studied. However, sperm are being produced and antibody may subsequently appear in spite of this treatment. c. Other methods of antibody suppression should be tested which may tell us more about the mechanism of immunization. The agents proposed are dexamethasone and anti-thymocyte globulin.